The present invention relates to cemented carbide bodies useful in tools for rock drilling, mineral cutting and in tools for road planing.
In U.S. Pat. No. 4,743,515 cemented carbide bodies are disclosed with a core of fine and evenly distributed eta-phase embedded in the normal alpha+beta-phase structure, and a surrounding surface zone of only alpha+beta-phase. (Alpha=tungsten carbide, beta=binder phase, e.g., Co, and eta=M.sub.6 C, M.sub.12 C and other carbides, e.g., W.sub.3 Co.sub.3 C). An additional condition is that in the inner part of the surface zone situated close to the core, the Co-content is higher than the nominal content of Co (with nominal is meant here and henceforth the weighed-in mount of Co). In addition, the Co-content in the outermost part of the surface zone is lower than the nominal and increases in the direction towards the core up to a maximum situated in the zone free of eta-phase. The zones free of eta-phase may, e.g., be created by adding carbon at high temperature to the surface zone of a body with eta-phase throughout.
Cemented carbide bodies according to U.S. Pat. No. 4,743,515 have given a positive increase in performance for all cemented carbide grades normally used in rock drilling. When drilling under such conditions that the outer layer of the cemented carbide is successively worn and ground away, the eta-phase-containing core, herein referred to as the eta-phase-core, is exposed. The risk for chipping and fracture is then increased due to the brittleness of eta-phase.
It has now been found that it is possible to obtain an .increased Co-content in the outer zone of the eta-phase-core and thereby essentially increase the toughness of the cemented carbide.